Inaccurate LUX/PPFD readings

[guccisimo85]

I will preface this by saying that measurements have only been taken on two iphone apps ... I do not yet have a light meter and I think the ones that measure PAR/PPFD are quite expensive...

My problem is that I have measured lux and PPFD in various locations. I seem to be getting correct PPFD numbers (50-70 by a bright window), but my lux measurements seem low for that PPFD (2500 lux max). I am even more confused because when I tried taking measurement in direct sunlight, I seem to only get max 250 PPFD and 10,000 lux.

Anyone have an idea what the problem may be?

Thanks!

EDIT: I have ordered a proper light meter (measures lux) which is supposed to arrive tomorrow!

[Ray]

I simply don't trust iphone meters.

Full sunlight at noon on the summer solstice at the equator is about 2000 µmol/sq. m/sec. Multiply by 54 to get lux.

[guccisimo85]

Originally Posted by Ray

I simply don't trust iphone meters.

Full sunlight at noon on the summer solstice at the equator is about 2000 µmol/sq. m/sec. Multiply by 54 to get lux.

Yes, I think it must be something to do with the inaccuracy of the phone apps. I will find out tomorrow when my light meter arrives. I imagine I can use that same conversion in reverse to calculate PPFD?

[guccisimo85]

Originally Posted by Ray

I simply don't trust iphone meters.

Full sunlight at noon on the summer solstice at the equator is about 2000 µmol/sq. m/sec. Multiply by 54 to get lux.

Any idea what the approximate conversion factor is for full spectrum T5 LED? I read that both warm and cool fluorescent lights have a conversion factor of 70, but not sure if LED’s would be different?

Thanks so much!

[Ray]

Unless you go fairly high-end on the meter, don't expect it to give reasonable readings for LED lamps.

Here's the Apogee Instruments conversion page.

[SouthPark]

Originally Posted by guccisimo85

Anyone have an idea what the problem may be?

The issue is a common one in the area of measurements. The associated word is 'uncertainty' or measurement uncertainty.

Even if you were to have an expensive device for measuring ------ that device needs to be properly calibrated. And some details (maybe from specifications) will also tell the user maximum expected error in measurement readings for particular ranges of light level (intensity, or whatever).

Calibration and information about device performance, plus having enough 'good quality' devices all providing similar values ----- helps to reduce the uncertainty in a measurement.

[camille1585]

Originally Posted by SouthPark

The issue is a common one in the area of measurements. The associated word is 'uncertainty' or measurement uncertainty.

Even if you were to have an expensive device for measuring ------ that device needs to be properly calibrated. And some details (maybe from specifications) will also tell the user maximum expected error in measurement readings for particular ranges of light level (intensity, or whatever).

Calibration and information about device performance, plus having enough 'good quality' devices all providing similar values ----- helps to reduce the uncertainty in a measurement.

At work we use 2 separate and fairly high end quantum light meters (Hortispec and a Licor) and even calibrated we're still getting readings with about 15% difference, and we measure 6 points per m2 to account for differences due to the LED bars... I can't even imagine what happens with lower quality meters.

[hypostatic]

I've tried a couple of the highly rated, accessibly priced, light meters from amazon... One of them gave impossible readings lol